Precipitation hardenable austenitic steel

ABSTRACT

The present invention relates to a stainless steel alloy, more precisely a highstrength stainless, precipitation hardenable, austenitic, stainless alloy, containing a well adjusted amount of aluminium and a high silicon content and which has the following composition (in weight-%): C 0-0.07 Si 0.5-3.0 N 0-0.1 Cr 15.0-20.0 Ni 7.0-12.0 Al 0.25-1.5 Cu 0&lt;Cu&lt;4.0 Mn 0-3.0 Mo 0-2.0 Ti 0-1.0 and the balance Fe together with normally occurring impurities and additives and a product that is reduced by cold working, especially drawing, without intermediate heat treatment, the strength of which increases by final heat treatment at 300° C. to 500° C. by not less than 14%, that shows a M d30 -value of between −55 and −100, a loss of force that is smaller than 3.0% at 1 N during 24 h and which is very suitable for use in spring applications, such as springs of round wire and strip steel and in medical applications, such as surgical and dental instruments.

TECHNICAL AREA OF THE INVENTION

The present invention relates to an austenitic stainless steel alloy,more precisely a high-strength precipitation hardenable austeniticstainless steel alloy containing a well balanced aluminium content and ahigh silicon content, a product which is reduced by cold working,especially drawing, without intermediate heat treatment, the strength ofwhich increases through final heat treatment at 300° C. to 500° C. bynot less than 14%, which shows a M_(d30)-value of between −55 and −100,a loss of force that is lower than 3.0% at 1400 N during 24 hours andwhich is very suitable for use in spring applications, such as springsof round wire and strip steel and in medical applications, such assurgical and dental instruments.

BACKGROUND OF THE INVENTION

On the market for stainless spring steel, the cold-worked austeniticstainless springsteels of type AISI 302 assume a dominating position.This is based on a combination of relatively good corrosion resistanceand a possibility to cold-work the material to a considerable strength,which is a prerequisite for a good spring material. Based on thecold-worked state, the mechanical properties may be increasedadditionally by means of a simple heat treatment. Steel of the type AISI631 is alloyed with aluminium in order to additionally enhance theincrease of strength at heat treatment. During cold-working, atransformation takes place from the annealed structure's principalconstituent of austenite to deformation martensite, which is harder thanthe phase from which it is formed. This quick deformation hardeningsimultaneously decreases the ductility of the material, and for thatreason soft annealing has to be executed at one or several steps in theproduction chain. This makes the production process more expensive, aswell as increases the risk of introducing surface defects in thematerial. For steel of the type AISI 631, the addition of aluminiumentails that the material tends to form ferrite in the structure duringsolidification after casting. The resulting austenite-ferritic structureand relatively low alloy content entails a quick deformation-hardening,which means that only moderate reductions are possible in order to avoidformation of cracks during the production process. Alternatively, steelsof the type AISI 304 and AISI 316 are used as spring steels. Thesesteels are higher alloyed and have a lower carbon content than steels ofthe type AISI 302 and AISI 631. This entails that a higher rate ofreduction can be allowed in this type of steel. The disadvantage ofthese steels is that the resulting product properties that are essentialfor a good spring function frequently are worse than for steels of AISI302 and AISI 631. One example of such a property is the resistance torelaxation, which describes the ability of a spring to retain springstrength over time.

U.S. Pat. No. 6,106,639 describes a Cr—Ni—Cu steel, which can be reducedstrongly between the annealings. In the exemplification it is indicatedthat a strength of 1856 MPa at a reduction of ε=3,41 (5,5 to 1 mm). Thisis compared with a specified strength according to the standard of 2050MPa. According to U.S. Pat. No. 6,106,639, a heat treatment has to beperformed to allow the alloy to attain strength values according to thisstandard. The alloy according to U.S. Pat. No. 6,106,639 contains copperas strength increasing element at heat treatment.

In U.S. Pat. No. 6,048,416, a Cr—Ni—Cu-steel intended for enhancement ofvehicle tyres in the form of high-strength steel wire is described. Inorder to attain the desired properties, the alloy according to U.S. Pat.No. 6,048,416, must composition-wise be within a stability intervalexpressed by a so-called JM value (JM=551−462×(C %+N %)−9.2×Si %−20×Mn%−13.7×Cr %−29×(Ni %+Cu %)−18.5×Mo %), which should be greater than −55but less than −30. In the alloy according to the invention, thecumulative logarithmic (ε=2*ln(S₀/S_(f))) rate of reduction is limitedto 4 as a maximum. This corresponds to a maximal area reduction at wiredrawing of 98%. Besides copper, the alloy according to U.S. Pat. No.6,048,416 contains no precipitation-hardening element.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a highstrength, precipitation hardenable, austenitic stainless steel alloycontaining a well-balanced amount of aluminium and a high siliconcontent, a product, which is reduced by cold-working, especiallydrawing, without an intermediate heat treatment, the strength of whichincreases by final heat treatment at 300° C. to 500° C. with not lessthan 14%, which shows shows a Md30-value of between −55 and −100, a lossof force that is lower than 3.0% at 1400 N during 24 hours and which isvery suitable for use in spring applications, such as springs of roundwire and strip steel and medical applications, such as surgical anddental instruments.

According to the present invention, these objects are attained by ahigh-strength, precipitation hardenable, austenitic stainless steelalloy, which contains (in weight-%): C more than 0 to 0.07 Si 0.5-3.0 N >0-0.1 Cr 15.0-20.0  Ni  7.0-12.0 Al 0.25-1.5  Cu 0 ≦ Cu ≦ 4.0Mn >0-3.0 Mo >0-2.0 Ti >0-1.0

Balance Fe and normally occurring impurities and additives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the loss of force of the springs after 24 hours ofmaterials according to the invention compared with AISI 302 and chargeno. 150725.

FIG. 2 shows the ultimate tensile strength of materials according to theinvention compared with AISI 302* (*—with intermediate heat treatment)and charge no. 150725.

FIG. 3 shows the ultimate tensile strength as a logarithmic function ofthe cumulative reduction rate of materials according to the inventioncompared with charge no. 150725.

FIG. 4 shows schematically a segment of a possible embodiment of anexpanding ring in a side view.

FIG. 5 shows in FIG. 5 a the ring seen from above. The ends are pressedagainst each other by the force F, in FIG. 5 b the ring is shown seenfrom the side, the ends being pressed against each other by the force Fand in FIG. 5 c a part of the expanding ring is shown that constitutes aflat spring element and how this is influenced by the force F.

FIG. 6 shows different embodiments for strip springs.

DETAILED DESCRIPTION OF THE INVENTION

The importance of the alloying elements for the present alloy is asfollows:

Carbon (C) has a high propensity to combine with chromium which meansthat chromium carbides is precipitated in the crystal grain boundaries,whereby the surrounding the bulk is depleted of chromium. Thus, at highcarbon contents the corrosion properties of the material deteriorate,problems also arise with embrittlement that foremost causes problem whenthe wire is shaped to springs. Therefore, the carbon content should beheld at as low a level as possible, being more than 0.0 weight-%, butmaximum 0.07 weight-%, preferably 0.05 weight-%, most preferably maximum0.035 weight-%.

Silicon (Si) has a ferrite-stabilising effect, which entails that too ahigh silicon content produces a two-phase structure. Therefore, thesilicon content should not exceed 3.0 weight-%. However, silicon is alsofavourable in that it contributes to a greater increase of strength atheat treatment of the cold-worked product. Therefore, the siliconcontent should not be lower than 0,5 weight-% and should be in the rangeof 0.5 to 3.0 weight-%, preferably between 0.5 and 2.5 weight-%, mostpreferably 0.5 to 1.5 weight-%.

Nitrogen (N) is an alloying element that together with aluminium formsnon-desirable brittle slags in the form of aluminium nitrides. Further,nitrogen increases the deformation-hardening at cold-working, which is adisadvantage in the present invention. Therefore, it is of highestimportance that the nitrogen content is held on as low a level aspossible, maximum 0.1 weight-%, preferably 0.05 weight-%.

Chromium (Cr) is a very important alloying element what concerns thecorrosion resistance of the material. This is due to the ability ofchromium to form a passive layer of Cr₂O₃ on the surface of the steel.In order for that passive layer to form, it is required that thechromium content exceeds approximately 12.0 weight-%, in addition, thecorrosion resistance increases with added chromium content. Anotheradvantage of chromium is that the austenitic structure of the materialis stabilized against transition to martensite at cold-working. However,chromium is ferrite-stabilising, and therefore the content should not betoo high. Therefore, in the alloy according to the present invention thechromium content should not be lower than 15.0 weight-% and not behigher than 20.0 weight-%, preferably be in the range of 16.0 to 19.0weight-%.

Nickel (Ni) is an alloying element that in a sufficient amountguarantees that the material gets an austenitic structure at roomtemperature. Furthermore, the ductility is improved with an increasednickel content. However, nickel is an expensive alloying element andhigh contents entail a slow deformation-hardening, which in its turnentails difficulties to attain a sufficient strength. Therefore, thenickel content should be within the range of 7.0 till 12.0 weight-%,preferably between 8.0 till 11.0 weight-%, most preferably within therange of 9.0 to 10.0 weight-%.

Aluminium (Al) is a central alloying element in the present invention.Aluminium is added as a precipitation hardening element in order toincrease the strength, which in turn influences the relaxationresistance. During precipitation-hardening at 350-500° C. of thecold-worked wire, precipitations in the form of β-NiAL are formed, whichimproves the mechanical properties unlike materials known until now.This effect is of highest importance when the wire is to be used assprings, the relaxation resistance of which has to meet very highrequirements. A disadvantage of aluminium is that it isferrite-stabilizing, for what reason the aluminium content should belimited to maximum 1.5 weight-%. However, in the light of theabove-mentioned, the aluminium content should be at least 0.25 weight-%and preferably be in the range of 0.4-1.0 weight-%.

Copper (Cu) is an alloying element that has two important properties.Firstly, copper is an austenite-stabilizing element and secondly copperdecreases the deformation-hardening of the material and entails improvedductility. Since the material has to withstand extreme reductionswithout intermediate annealings, the copper content has to be as high aspossible. However, with an increasing copper content, the risk ofunwanted precipitations increases, which decreases the ductility of thematerial. Therefore, the copper content should be in the range of0≦Cu≦4.0 weight-%, preferably between 2.0 to 3.5 weight-%, mostpreferably between 2.4 to 3.0 weight-%.

Manganese (Mn) has similar effect as nickel, both with regard to formingaustenite at setting as well as stabilizing the same againsttransformation into martensite at cold-working. However, manganeseincreases the deformation-hardening, which nickel does not. This resultsin a faster deformation-hardening and diminishes the greatest possiblereduction rate between the annealings. Therefore, the manganese contentshould be more than 0.0 weight-%, but being limited to maximum 3.0weight-%, preferably to maximum 1.0 weight-%.

Molybdenum (Mo) is a ferrite-stabilizing element that has a stronglyfavourable effect on the corrosion resistance in chloride environments.Established PRE (Pitting Resistance Equivalent) formulas allocatemolybdenum a factor of ≈3 in comparison with the effect of chromium.However, a high molybdenum content stabilises the ferrite phase in thesteel. Further, there is an increased risk of precipitation ofintermetallic phases, such as sigma phase. Therefore, the molybdenumcontent should be more than 0.0 weight-%, but limited upwards to 2.0weight-%.

Titanium (Ti) is, like aluminium, a precipitation-hardening element thatis added in order to increase the strength, which in turn influences therelaxation resistance. Furthermore, titanium together with silicon givesa strong heat treatment effect already at low contents of titanium.However, titanium is strongly ferrite-stabilizing, for what reason thecontent should not be too high. Therefore, the titanium content shouldbe more than 0.0 weight-%, but being limited up to 1.0 weight-%,preferably maximum 0.75 weight-%.

Description of the Test Procedure

The test materials were produced by melting in a high frequency furnace.Subsequently, all test ingots were fully ground before they were forged.Forging was performed on the ingot to 103×103 mm length in stock. Theheating temperature was in the range between 1240° C. and 1260° C. Theholding time at full temperature was 1 h. At the subsequent blanktreatment, the blanks were fully ground and ultrasonically tested.

The wire rod in the dimension range of Ø 5.50 mm-Ø 5.60 mm was producedby warming the blanks to 1200° C.-1240° C., whereupon they were rolledto final dimension and then cooled by water quenching. The hot-rolledwires were then cold-worked by drawing in a conventional drawingmachine.

The chemical composition, in weight-%, of the alloys in the test programand reference materials are given in Table 1. TABLE 1 Chemicalcomposition (in weight-%) 1 2 3 4 5 6 7 AISI 302 150725 C 0.023 0.0210.023 0.027 0.033 0.024 0.019 ≦0.12 0.011 Si 0.96 1.46 1.37 0.59 0.961.45 0.88 ≦2.0 0.51 N 0.021 0.020 0.019 0.018 0.020 0.022 0.034 ≦0.10.012 Cr 16.45 16.35 16.42 16.46 16.73 16.74 17.40 ≧16.0 17.44 ≦19.0 Ni9.68 9.61 9.73 9.82 9.02 9.38 9.32 ≧6.0 9.48 ≦9.5 Al 0.42 0.93 0.81 0.830.44 0.96 0.71 — <0.003 Cu 2.99 2.97 2.98 3.00 2.48 3.04 2.95 — 3.02 Mn0.68 0.93 0.73 0.70 0.93 0.95 0.86 ≦2.0 0.66 Mo <0.01 <0.01 <0.01 <0.01<0.01 0.17 0.07 ≦0.80 0.16

The strength of the alloys in cold-worked state and after heat treatmentat uniaxial tensile testing is seen in Table 2, where the ultimatetensile strength corresponds to the maximum value of the load in theelongation-load diagram. All alloys have been reduced to a logarithmiccumulative degree of reduction of ε=3.95 (corresponding to an areareduction of 98%) without intermediate annealing. AISI 302 could not becold-worked to ε=3.95 without crack formation, because of which anannealing operation had to be carried out before drawing to finisheddimension. However, all alloys have the same wire diameter.

The heat treatment was accomplished with the same purpose as for springsteel of the type AISI 302, when an increase of the mechanicalproperties is obtained. Thereby, several important spring properties,such as, for example, the relaxation resistance, are influenced but in astronger way than known hitherto. TABLE 2 Ultimate tensile strengthbefore and after heat treatment. Ultimate tensile Ultimate tensilestrength after strength after cold working heat treatment Heat treatmentCharge no. [MPa] [MPa] effect [%] 1 2014 2298* 14.1 2 2132 2496* 17.1 32136 2442* 14.3 4 1942 2502* 28.8 5 2162 2482** 14.8 AISI 302 2140 2370*10.7 150725 1760 1953* 11.0*Heat treatment time = 1.5 h, Heat treatment temperature = 350° C.**Heat treatment time = 1.0 h, Heat treatment temperature = 480° C.

For evaluation of the relaxation resistance, springs of the typecylindric helical springs not having lined-up turns were produced. Thetest results are seen in Table 3. TABLE 3 Spring dimensions Wirediameter (D_(t)) 0.762 mm  Inner diameter of the springs 6.84 mm Averagediameter (D_(m)) of the springs  7.6 mm Pitch 1.52 mm Number of turns(N_(v)) 50.5

The spring force (F) and the total spring suspension (ft) weredetermined at room temperature by means of a force versus load curve.Subsequently, the spring constant (C) and shear modulus (G) werecalculated by means of equation 1 and 2.C=(F*N _(v))/f _(t)  Equation 1.G=(8*F*Nv*D _(M) ³)/(ft*D _(t) ⁴)  Equation 2.

The relaxation test was accomplished by loading blued springs with aconstant load. The load was read each minute under the first fiveminutes and then the number of read-outs was cut down. Each test wasstopped after twenty-four hours. Springs from the respective charge wereloaded initially on four different levels. The relaxation was calculatedby means of equation 3 and the results are summarised in FIG. 1.R=((F ₁ −F ₂)/F ₁)*100  Equation 3whereR=RelaxationF₁=Initial loadF₂=Load at a given time

In FIG. 1 it is seen that the alloy having a very low aluminium content,i.e. charge no 150725 relaxes considerably stronger than the alloys inthe test program, which all have aluminium as an active alloyingelement. Furthermore, all alloys in the test program have an equivalentor better relaxation resistance than AISI 302.

M_(d30)/Nohara shows the temperature where at a rate of cold reductionof 30%, 50% of the austenite in the steel is transformed totransformation-martensite. A higher value for the temperature indicates,that the structure is more stable (more disposed to form martensite) andleeds to a higher rate of cold-deformation in the steel.

The M_(d30)-value according to Nohara is calculated by the formula:M_(d30)/Nohara=551−462×(C+N)−9,2×Si−8,1×Mn−13,7×Cr−20×(Ni+Cu)−18,5×Mo−68×Nb−1,42×(ASTMgrain size−8).

Table 4 shows the results for the test charges 1 to 7. It hassurprisingly shown that a steel with the composition according to thepresent invention attains the best heat treatment effect atM_(d30)-values of between −55 and −100 and the highest increase inultimate tensile strength after solely cold working without intermediateheat treatment. TABLE 4 M_(d30)/Nohara Charge nr. Ni— ekvivalentM_(d30)/Nohara [° C.] 1 23.60 −76.5 2 23.65 −77.7 3 23.64 −80.5 4 23.50−78.2 5 23.19 −52.6 6 23.79 −80.8 7 23.99 −82.8Description of Preferred Embodiments

In the following, some embodiments of the invention are described. Theseare intended to illustrate the invention, but not limit it.

The steel according to the present invention is subjected to a strongcold deformation. It can be shaped to different cross-sectiongeometries, for example, round, oval wire, profiles of differentcross-sections, for example, rectangular, triangular or more complicatedembodiments and geometries. Round wire may even be flat-rolled.

EXAMPLE 1 Springs of Round Wire

As been described above, springs of wire made from the alloy accordingto invention are wound. These springs have good spring properties in theform of relaxation, i.e. the retention of spring force under a longperiod and are advantageously used in typical spring applications, suchas, for instance, springs in locking applications, i.e. mechanical partsin the locking device, springs in aerosol containers, pens, especiallyball point pens, pump springs, springs in industrial looms, springs inthe vehicle industry, electronics, computers and fine mechanics.

EXAMPLE 2 Springs of Strip Steel

For plane torsion springs, the torque is a decisive quantity. The torquecan be expressed as$M = \frac{E*I*2*{\pi\left( {n - n_{0}} \right)}}{L}$where:M=the torque of the springI=moment of bending inertia (b*t³/12)B=spring strip widthT=spring strip thicknessL=extended spring lengthno=number of turns at free spring (unmounted)n=number of working turns

In order to increase the torque at a given spring geometry, a so-calledreverse winding may be accomplished. At a so-called “resilient” winding,the spring is preformed by being wound in a direction opposite theworking direction. Then a heat treatment of the spring takes place,after which it is wound-in in the opposite direction in the springhousing. At so-called “cross curve” winding, the strip is formed on atack, after which heat treatment takes place. Then the spring is woundin the opposite direction into the spring housing. By means of thisprocedure, a lower and sometimes even a negative value of n₀ can beobtained in comparison with a singly wound spring, see FIG. 6. Due tothe very good increase of strength at heat treatment, the alloyaccording to the present invention is very suitable for use as torsionsprings, where high torque and good relaxation resistance is required.

EXAMPLE 3 Expander Wire

An expander is a bit of wire, which is corrugated and shaped to a flatspring connected in series. This spring is used, for instance, in orderto regulate the pressure of the oil scraper rings against the cylinderwall in an internal combustion engine. A typical expander for car motorsis seen as the corrugated wire between two piston rings. A possibleembodiment of such a corrugated ring is shown schematically in FIG. 4.

A drawback of motor-driven vehicles today is the great energyconsumption that is necessary in order to give the vehicle the desiredperformance thereof. The easiest ways to achieve a reduced energyconsumption is, among other things, to diminish the internal friction ofthe drive and to reduce the total mass of the vehicle. The piston coreaccounts for more than half of the friction of a motor. Therefore, it isa continuous aim to improve the material and precision of the rings,pistons and cylinder walls with the purpose of reducing tare weights andbearing pressure. The expander is the spring that regulates the pressureof the oil scraper rings against the cylinder wall and thereby also oilconsumption and part of the internal friction of a motor. The load ofthe expander wire consists of the force F, as shown in FIGS. 5 a to 5 c.

For a flat spring, where the load is applied at an angle of 90° to themaximally loaded back, the following relation applies: σ_(Umax) Allowedmaximum load in the back of the spring F the loading force which isdetermined by the length of the expander wire in relation to the pistondiameter T Thickness of the wire B Width of the wire E Modulus ofelasticity of the wire material s Suspension travel, how much theexpander is deformed R The bending radius in each spring element${(1)\quad\sigma_{\max}} = \frac{6\quad{FR}}{{BT}^{2}}$${(2)\quad s} = \frac{42\quad R^{3}F}{{EBT}^{3}}$ (3) the combination of(1) and (2) gives:$B = {\frac{42\quad R^{3}F}{{EsT}^{3}} = {\left. \frac{6\quad{FR}}{\sigma_{\max}T^{2}}\Rightarrow T \right. = \frac{7R^{2}\sigma_{\max}}{Es}}}$

Expression (3) shows that the wire thickness that is required for agiven property depends on the design of the expander. If the allowedtension of the material is increased, a smaller bending radius can beallowed, which is of great interest since rings of smaller types can bemanufactured. The possibility of being able to manufacture smaller ringsbecomes more and more important since the demand for small motorsincreases as the environmental requirements are raised.

Another way to see the benefit of a higher strength in the expandingring is by making an energy consideration according to the reasoningbelow. A Elastic energy K Material-use constant E Modulus of elasticityV Effective volume of the spring (how much of the material of the springthat is working) σ Applied tension${(4)\quad A} = {{VK}\quad\frac{\sigma^{2}}{E}}$

Expression (4) shows that a certain elastic energy for given modulus ofelasticity is a function of the specific volume, material use andallowed maximum tension. An increased maximal allowed tension increasesas a rule the material-use constant, which in combination gives a majorimpact on the required specific volume. Thus, it is possible to diminishthe material volume increased allowed tension for retained level ofelastic energy.

To form an expanding ring to the complex form thereof is only possiblewith soft materials. The workability is the primary reason for stainlesssteel being used at all. For the function of the expander, however, thetensile yield limit and ultimate tensile strength are at least asimportant as in all spring applications. This has earlier been a stateof contradiction difficult to manage. By using the steel according toinvention, the material may be formed in a relatively soft state so asto later be heat treated in the finished form, whereupon the desiredspring properties are obtained by precipitation hardening.

EXAMPLE 4 Flat Wire

This embodiment according to the present invention is used especially inapplications that make great demands on the relaxation properties of thesteel, since it should resist a force without being preformed. Thismakes the steel especially suitable for use as, e.g., wire forwindscreen wipers, where a good punchability of the starting materialshould be combined with a good relaxation resistance of the finishedproduct.

EXAMPLE 5 Round and Flat Wire as Well as Strip Steel for MedicalApplications

Wire, manufactured from the alloy according to invention may even beused in medical applications, for instance, in the form of dentalinstruments as files, such as root canal files, nerve extractor and thelike, as well as surgical needles. Flat-rolled wire of the steelaccording to invention may advantageously be used for the production ofdental and surgical instruments.

All these applications have in common that they have complicatedgeometries, which are produced by grinding, bending, and/or torsionadvantageously before the last heat treatment and which then get astrong increase of the mechanical properties, i.e. a high breakingstrength in combination with a good ductility.

1. Product, manufactured from a high-strength austenitic stainlessalloy, that it is precipitation hardenable and has the followingcomposition (in weight-%): C more than 0 to 0.035 Si 1 N>0-0.1 Cr about17.5 Ni 9.0-10.0 Al 0.7-1.0 Cu 3 Mn 0.7-1.0 Mo>0-2.0 Ti>0-1.0 and thebalance Fe together with normally occurring impurities and additives,said alloy having been reduced by cold-working without an intermediateheat treatment followed by a final step of heating to a temperature of300° C. to 500° C. to increase strength by at least 14%.
 2. A product ofclaim 1, which has loss of force which is smaller than 3.0% at 1400 Nduring 24 h.
 3. A product of claim 1 in the form of wire, profilesand/or strip.
 4. A product of claim 1 in the form of springs of roundwire and strip steel.
 5. A product of claim 1 in the form of surgicaland dental instruments.
 6. A product of claim 1, which has loss of forcewhich is smaller than 3.0% at 1400 N during 24 h.
 7. A product of claim1 in the form of wire, profiles and/or strip.
 8. A product of claim 1 inthe form of springs of round wire and strip steel.
 9. A product of claim1 in the form of surgical and dental instruments.
 10. Product,manufactured from a high-strength austenitic stainless alloy, that it isprecipitation hardenable and has the following composition (inweight-%): C more than 0 to 0.035 Si 1 N>0-0.1 Cr 17 Ni 9.0-10.0 Al0.7-1.0 Cu 3 Mn 0.7-1.0 Mo>0-2.0 Ti>0-1.0 and the balance Fe togetherwith normally occurring impurities and additives, said alloy having beenreduced by cold-working without an intermediate heat treatment followedby a final step of heating to a temperature of 300° C. to 500° C. toincrease strength by at least 14%.